Electrical breakdown device of the cavity resonator type



Oct. 24, 1950 A. M. STONE ELECTRICAL BREAKDOWN DEVICE 0F THE CAVITY RESONATORTYPE Filed May 20, 1943 Vf/ ///a FIG. I

FIG. 2

ATTORNEY Patented Oct. 24, 1950 ELECTRICAL BREAKDOWNV'DEVICE OF THE CAVITY RESONATOR TYPELI Albert- M. Stone, Brookline, Mass, assi mesne assignments, to the Unitedv tate's jof' America as represent ed by the Secretaryofthe Application May 20, 1943, Serial No, 487,775

. 13 Claims (cr s s-) This invention relates to protective electric breakdown devices and particularly to improvements in such devices for increasing the degree of protection alforded therewith to sensitive apparatus such as crystal detectors of radio receivers, and especially improvements in such devices directed to reducing the losses occurring there-' in, increasing the tuning range, and other purposes. i

The invention relatesparticularly to the class of protective electric breakdown devices described in the patent application of James L. Lawson, Serial No. 479,662, filed March 18, 1943, and entitled Protection of Receiver Against Overload, including a breakdown gap associated with an electrical resonator and provided with means for maintaining a suitable low-pressure gaseous atmosphere in the neighborhood of said breakdown gap in order to promote breakdowns of said resonator when electrical oscillations exciting the resonator exceed a predetermined intensity. Difficulties have been experienced with devices in this class, especially in connection with operation at relatively high power levels, on account of the passage through the device of a sharp pulse 'of electrical energy at the initiation of the breakdown, and also because of a tendency of a glow discharge to take place in the neighborhood of the input coupling to the device, and for other reasons. I have found a particularly desirable gas mixture for the atmosphere of the breakdown gap for minimizing difiiculties of the first-mentioned type. In addition, I employ certain other improvements described below which cooperate with the improvement just mentioned to produce protective electric breakdown devices capable of improved performance. Objects of this invention include the provisions of a protective electrical breakdown device capable of providing an increased degree of protection to associated apparatus, the reduction ofv undesired losses in protective electrical breakdown devices, improvement of the reliability of operation of such devices, and the like.

. The invention is illustrated in the accompanying drawing, in which Fig. 1 is a longitudinal cross-section of one form of protective electrical breakdown device incorporating improvements of this invention and Fig. 2 is a transverse crosssection of another form of protective electrical breakdown device improved in accordance with this invention.

Protective electric breakdown devices are most commonly used in ultra-high-frequency locating and detecting systemsof the radio-echo type, and particularly such systems in which a transmitter and a receiver .are operated in con-- nection with a single antenna system, the transmitter being intermittently operated to produce a series of short pulses. .The pulses of ultrahigh frequency energy produced by the trans-- .mitter are such thatfor substantially the entire durationof the pulse the transmitter is operat- The electrical breaking at a high power lever. down device may then be associated between the input to thereceiver and the antenna in such a manner that when-the transmitter operates, a breakdown occurs in the device, reducing power transfer through the device and thus preventing more than a small amount of power from reaching the-receiver while the transmitter is operating. When the transmitter is not operating, however, relatively efficient power transfer is possible through the device, so that the receiver may be ellectivelyconnected to the antenna in order to respond to echoes of transmitted pulses which may arise in the interval between transmitted pulses.

In order to maintain a suitable partial vacuum in: the; neighborhood ,of the breakdown gap of the. electricalbreakdown device, a glass envelope, usually cylindrical in shape is generallyprovided about the electrodes forming the breakdown gap. Ifthisenvelope is too close to the gap, the discharge takes place, right out to the glass en-.

velope itself-and sometimesmetallic particles from the electrodes are sputtered onto the glass, greatly reducing the 'efifectivenessof the resonatorrby introducing losses. ;If, on the other hand, the glass envelope is made sufliciently great in diameter to beat a considerable distance from the breakdowngapgin some devices in which the breakdown gap tothe glass wall of the vacuum maintaining envelope, or to the top or bottom metal plates closing the cavity, so that metal particles may be sputtered upon the glass, thus reducing the'effectiveness of the resonator of the device by introducing undesired losses.

An important feature of, this invention, there-' fore,'is the location of the glass portions of the vacuum maintaining means of the device. An-

other important featureisthe use of an at:

The device shownin Fig. 1 includes aresonator shown generally at 3 comprising a cylindrical wall and disk-like end walls 5 and 6. The wall 6- is in the form of a flexible diaphragm-the-axial position of the center of which is controlled by a threaded rod '1 which is adapted to be manipulated by the knob 8 mounted'in the .ball bearings:-

9. The end walls 5 and 6 are respectively provided with central conical op'posedelectrodes l and l I defining the breakdown gap I2. The axial movement of the electrode ll, ascontrolled by the knob 8, is adapted to tune the resonator 3 by varying the capacitance across the gap [2. In'

order that a considerable tuning range may be obtained by this method withoutlimiting thedegreeof protection afforded at the high frequency end of the tuning range below the degree of'protection desirable, as a result of excessive spacing between the electrodes [0 and II when thecapacitance across the gap I2 is reduced by separatingthese electrodes, the opposed extremitiesof the electrodes l0 and H are serrated to provide a plurality of opposed sharp points. These sharp points are adapted to facilitate the breakdown of the device even with fairly large spacing between the electrodes 0 and l I, by concentrating the electric stress in the neighborhood of the points.

An additional electrode [4 is provided within the conical electrode H], which is hollow, inorder to provide a small amount of ionization not far from the gap 12, the ions formed being adapted to wander into the gap [2, and to promote the occurrence of breakdown when theresonator 3 isexcited. The electrode I4 is brought out of the partially evacuated resonator 3 through aglass seal 15 which also serves to insulate the electrode M from the resonator structure. The electrode ll -is connected with a source of relatively steady voltage through a wire IS. The voltage applied.

may-be a voltage of 1000 volts through a five megohm current-limiting resistor. proper selection of the gas atmosphere in the resonator 3 and of the polarity of'the voltage of the electrode 14, it is possible to reducethe electrode voltage to 400 or even 225'volts without important change in the degree of protection This reduction of the afiorded by the device. electrode voltage is particularly advantageous for apparatus for installation in aircraft where highvoltage' power supplies are. preferably used sparingly.

For the gaseous atmosphere in the dischargegap I'prefer mixtures of 80% argon and 20% helium at a pressure of 8-10 mm. of mercury. In general I find that argon-helium mixtures with ahelium content of -30%' at pressures of 5-20 mm. are advantageous and distinctly superior to.

advantage that the moisture content may change.

with temperature and mayalso change with time, apparently becoming occluded orthe like so that the characteristics of a device employing a moist With the.

atmosphere may tend to change gradually while the device is in service.

For best reduction of the peak power passed by the device, which, as noted before, is the power of the short pulse passed at the initiation of the breakdown, a negative voltagezupon the electrode I4 is desirable. For-reduction'of the deionization time, however, a positive voltage on the electrode M- is more efiective, the difference in effect being particularly marked when an atmosphere is used of. the composition above described. The deionization time characteristics of the gas mixture appears to be quite sensitive to impurities in the gases. Negative or positive voltage for the electrode l4 may be selected according to whether peak power passed or deionization time is more important in the particular apparatus in question- If improvements, such as the use of the helium-argon atmosphere and the provision of the serrations on the electrodes 10 and I i provide by themselves enough reduction of the peak powerpassed by the device, the device may be.

operated with a positive voltage on theelectrode l4in order to obtain rapid deionization, which is particularly important in radio-echo location systems intended for determining the position of relatively close objects, such as objects less than 1000"yards away. Even withpositive polarity, of

the electrode I4, reasonable electrode voltages, such as 500 volts or less,ysuflice to keep the peak power passed through the device within usually satisfactory limits.

Another" possible method of, obtaining, both rapid deionization and'low peak power output of the device when a helium-argon mixture is used for the gaseousatmosphere as herein described, is to employ two electrodes each like the electrode Within the conical electrode structure l0. One of these two electrodes could then be provided with a voltage negative with respect to the resonator structureand the other of these electrodes could be impressedwith a voltage positive with respectto the resonator'structure. The efiect of such a second electrode may be partially achieved by providing sharp projections on the inside surface of the hollowconica-l electrode [0, adapted to produce points of concentrated electric stress.

Coupling to the resonator 3 is accomplished by the-loop l8 which couples the resonator to the.

coaxial-conductor transmission line l9, and by the-window 20 which couples the resonator to the coaxial-conductor transmission line 2!. Either one of these coupling arrangements may be used as" the input and the other as the output. For the input coupling the type of coupling should be chosen which produces the least glow in the partially-evacuated portion of the device nearest to the coupling arrangement. Both methods of coupling shown in Fig. l are adapted to provide relatively little or'noglow in the neighborhood 'of the glass vacuummaintaining means which are located atv 23 and 24. If the input coupling is provided through the window 20, the coaxialconductor transmission line 2| may lead in one direction to a transmitterand in the other direction'to an antenna. It is believed that input coupling through the loop I8 is somewhat preferable to input coupling through the window 20 because of the greater degree of isolation providedxfrom the evacuatedportion of the device. When, accordingly, the loop I8 is used for input coupling, the transmission line [9 may be connected to a transmission line junction from which transmission lines lead to a transmitter and an antenna". The'transmission-line 2| then may end of the transmission line 2| being closed ofi ,4

by a suitable reflecting termination or stub sup port, or being used to connect to the output of a local oscillator. If desired, both input and output coupling may be by loops such as the loop |8,: or both may be by windows such as the window The important thing in this connection with 20. which this invention is concerned is that the glass vacuum sealing means, which on account of their g shape may be referred to as hats, should extend inwardly as shown on Fig. 1 towards the center of the resonator in order that the space adjoining the coupling device in each case may be at.

tric breakdown devices having certain features in common with the device in Fig. 1. For this device, as in the case of Fig. 1, an atmosphere of about 20% helium and 80% argon at a pressure of about 8 to 10 millimeters of mercury is preferred. Although in the device shown in Fig. 2 the tuning is not accomplished by varying the gap spacing, serrated electrodes having teeth as shown at 28 are provided at the breakdown gap in order to provide prompt breakdown and therefore to reduce as much as possible the power level of the sharp pulse which is passed through the device at the initiation of the breakdown.

The breakdown device is shown in Fig. 2 in a median transverse cross-section. One of the conical electrodes forming the discharge gap is shown at 30. The cylindrical wall of the resonator 3| is shown at 32. Tuning is accomplished in this device by means of cylindrical plugs 33 and 34 which are adapted to be adjustably inserted into the resonator 3 I. The adjusting mechanism is not shown, the plugs 33 and 34 and their associated structure being broken oif in Fig. 2 for simplification of illustration. The plugs 33 and 34 may be simply threaded into the structure of the resonator 3| or more elaborate arrangements known to the art for providing for radio frequency by-passing in order to reduce losses may be employed.

Input coupling is provided by a loop 36 connected to a coaxial-conductor transmission line 31. An output coupling is provided by a loop 38 connecting to a coaxial-conductor transmission line 39. In order to reduce the tendency of a glow discharge to occur in the proximity of the loop 36, the glass vacuum-maintaining means 40 is provided in an eccentric position with respect to the electrode 30. In this manner the partially evacuated portion of the resonator 3| is kept at a substantial distance from the loop 36 and at the same time the disadvantage of having the glass envelope too close to the discharge of the envelope 40, which is cylindrical in shape,

the vacuum-maintaining means of the resonator 3| follows the practice that has by this time become conventional, which is to say that the glass cylinder 40 is sealed to. the disk carrying the conical electrode 30, forming the top and bottom 6? of the resonator 3| and being adapted to/be clamped to the cylindrical wall 32 of thesaid resonator.

resonator 3| if the conical electrodes 30 have a central aperture. The electrode 30 shown in- Fig. 2 is provided with such a central aperture to allow communication between the portion of the resonator 3| enclosed by the glass envelope 40 and the space enclosed by the conical electrode 30 in which an electrode 45, corresponding in function to the electrode M of Fig. 1, is lo;- cated.

In addition to the eccentric arrangement of the glass envelope 40 with respect to' the electrode 30, other means may be provided for the reduction of the tendency to form a glow discharge in the neighborhood of the input coupling loop, such means for example, being shown in Fig. 2 by the electrostatic shield strap 46 provided around the loop 36 at right angles thereto. The shield strap 46 is simply a narrow strip of copper or a piece of copper wire mounted as shown. It is located so as to cross about in front of the middle of the loop 36. Such an arrangement has been found to produce some reduction of the tendency to form a glow discharge in the part of the evacuated enclosure of which is nearest to the input loop, but, if desired, this arrangement may be dispensed with, especially if the location of the glass envelope 40 as shown on Fig. 2 is suificient to avoid such undesired glow discharges.

What I desire to claim and obtain by Letters Patent is:

1. A protective electrical breakdown device for the protection of ultra-high-frequency radio ap paratus against overload, which device includes an electrical resonator and a discharge gap disposed Within said resonator, said discharge gap being formed by electrodes constituting apart of said resonator and having serrated extremities, an additional electrode located within one of said first mentioned electrodes in a space communie eating with said discharge gap and shielded from said resonator by the electrode within which it is located, means coupled to said additional electrode for applying to said additional electrode a potential of less than 400 volts with respect to said resonator, means maintaining, in the portion of said resonator including said discharge gap and said additional electrode, an atmosphere consisting substantially entirely of argon and hell-um, the proportion of helium being between 5 and 30%, at a pressure between 5 and 20 mm. of mercury.

2. Apparatus in accordance with claim 1 in which said means for maintaining said atmosphere includes recessed cup-like glass wall members sealed to said resonator circumferentially of openings in said resonator through which electrical coupling is provided to said resonator and also a glass seal for said additional electrode, said recessed glass members being adapted because 'of their recessed shape to confine said atmosphere to portions of the resonator sufficiently distant from coupling means of said resonator to avoid glow discharges impinging on said glass members.

3.'Apparatus in accordance with claim 1 in which said means for maintaining said atmosphere includes a tubular glass partition enclosing said gap and the electrodes formingsaid gap and sealed to walls of said resonator, said apparatus further comprising an input coupling means disposed in a wall of said resonator said parti- Additional glass envelopes may be; provided above and below the disks outside of the than having its central axis displaced from the: central axis of said electrodes forming said gap-v inv a direction opposite pling means.

4. A protective electrical breakdown device for.

to that of. said input couthe protection of ultra-high frequency radio ap paratus against overload, whichdevice includes an electrical resonator, a pair of electrodes form-.

:5. A protective electrical breakdown device as in claim 4, said device further comprising an electrostatic shield strap disposed at right angles to the'plane of said input coupling loop and electrically connected to a wall of said resonator.

6. .A protective electrical breakdown device for the protection of ultra-high frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator, coupling loops extending .within said resonator, means maintaining a predetermined atmosphere in the region of said discharge gap, said lastmentioned means including recessed cup-like glass wall members sealed to said resonator circumferentially of openings in said resonator, said recessed glass wall members being adapted because of their recessed shape to confine said atmospheres to portions of the resonator suficiently distant from said coupling loops to avoid glow discharge impinging on said glass wall members.

'7. A protective electrical breakdown device for the protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator, said electrodes being formed with serrated extremities, an additional electrode located within one of said first mentioned electrodes in a space communicating with said discharge gap and shielded from said resonator by the electrode within which it is located, means coupled to said additional electrode for applying to said additional electrode a predetermined potential with respect to said resonator and means maintaining, in the portion of said resonator including saiddischarge gap and said additional electrode, a gaseous atmosphere at a pressure less than atmospheric.

8. A protective electrical breakdown device for the protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap' disposed within said resonator, said electrodes bein formed with serrated extremities,

an additional electrode'locatedwithin one of said onator and means maintaining, in the portion of said resonator including said discharge gap and said additional electrode, a gaseous atmosphere;

at a' pressure less than atmospheric, said lastmentioned means comprising recessed, cap-like, dielectric wall members sealed to said resonator circumferentially of openings in said resonator through which electrical coupling is provided to said resonator, said recessed dielectric wall mem hers-being adapted because of their recessed shape to confine said atmosphere to portions of the resonator sufficiently distant from coupling means of said resonator to avoid glow discharges impinging on said dielectric wall members.

9. A protective electrical breakdown device for protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator and means maintaining in the portion of said resonator including said discharge gap a gaseous atmosphere at. a pressure less than atmospheric, said lastmentioned means comprising a tubular dielectric partition enclosing said gap and the electrodes forming said gap and sealed to walls of said resonator, said apparatus further comprising input coupling means disposed in a wall of said resonator, said partition having its central axis displaced from the central axis of said electrodes forming said gap in a direction opposite to that of said input coupling means.

10. A protective electrical breakdown device for protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator and means maintaining in the portion of said resonator including said discharge gap a gaseous atmosphere at a pressure less than atmospheric, said lastmentioned means comprising recessed, cup-like, dielectric wall members sealed to said resonator circumierentially of openings in said resonator through which electrical coupling is provided to said resonator, said recessed dielectric members being adapted because of their recessed shape to confine said atmosphere to portions of the resonator sufficiently distant from coupling means of said resonator to avoid glow discharges impinging on said dielectric members.

11. A protective electrical breakdown device for protection; of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gapdisposed within said resonator and means maintaining in the portion of said resonator including said discharge gap a gaseous atmosphere at a pressure less than atmospheric, said lastmentioned means comprising a tubular dielectric partition .of circular cross-section enclosing said gap and the electrodes forming said gap and sealed to walls of said resonator, said apparatus vfurther comprising input coupling means disposed ina wall of said resonator, said partition having its central axis displaced from the central axis of saidelectrodes forming said gap in a direction .opposite to that of said input coupling means.

12. A protective electrical breakdown device for protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator and means maintaining in the portion of said resonator including said discharge gap a gaseous atmosphere at a pressure other than atmospheric, said lastmentioned means comprising a tubular dielectric partition enclosing said gap and the electrodes forming said gap and sealed to walls of said resonator, said apparatus further comprising input coupling means disposed in a wall of said resonator, said partition having its central axis displaced from the central axis of said electrodes forming said gap in a direction opposite to that of said input couplin means.

13. A protective electrical breakdown device for protection of ultra-high-frequency radio apparatus against overload, which device includes an electrical resonator, a pair of electrodes forming a part of said resonator and defining a discharge gap disposed within said resonator and means maintaining in the portion of said resonators including said discharge gap a gaseous atmosphere at a pressure other than atmospheric, said lastmentioned means comprising dielectric wall members sealed to said resonator, said dielectric wall members cooperating with said resonator to completely enclose said gap and the electrodes forming said gap, said apparatus further comprising input coupling means disposed in a wall of said resonator, said dielectric wall members being shaped to enclose a volume of atmosphere the center of which is displaced from the central axis 10 of said electrodes in a direction opposite that of said input coupling means, said wall members because of their shape confining said atmosphere to portions of the resonator sufficiently distant from said coupling means of said resonator to avoid glow discharges impinging on said dielectric wall members.

ALBERT M. STONE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

